Punctuated Equilibrium and the Validation of Macroevolutionary Theory 867
some important hominid-bearing strata). They did document one or two cases of
gradual change, notably an increase in third molar length for Mesochoerus
limnetes. But the clade includes 16 species during this short period of no more than
4 million years, 8 of which arise by punctuational cladogenesis even in White and
Harris's own diagram (1977, p. 14). The authors' comments, unwittingly I suspect,
frequently point to the domination of evolutionary history in this clade by
cladogenetic events and their consequences. They write (1977, p. 14), for example,
that "Metridochoerus underwent a substantial adaptive radiation during the early
Pleistocene, and at one point four distinct metridochoere species existed
contemporaneously."
Many of the best invertebrate examples fall into the same category of unique
and endemic taxa confined to isolated places, and therefore forming, by strong
inference, a complete and coherent phylogenetic unit—as in Williamson's study
(1981), cited several times previously, of speciation in pulmonate snails from
separated African lakes. In another example from a famous sequence of much
greater temporal extent, Geary (1990, 1995) studied the evolution of melanopsid
gastropods in the Middle to Late Miocene beds (spanning 5 to 10 million years) of
the Pannonian Basin in Eastern Europe. In one case of gradualism, following a
much longer interval of at least 7 million years in stasis for the ancestral form,
Melanopsis impressa transformed to M. fossilis by directional increases in shell
size and shouldering over a two million year interval. However, within the same
Pannonian Stage, at least six new melanopsid species arose by punctuation: "their
first appearances are abrupt, and preceded by no intermediate forms" (Geary, 1995,
p. 68). Geary (p. 69) regards the stratigraphic resolution as "not particularly good,"
but still fixes the origin of these species to within "tens of thousands of years"—a
clear punctuation by the criterion of scaling against average species duration in
stasis within the clade. Figure 9-27 (from Geary, 1995, p. 68) depicts Geary's
results for this geographically isolated evolutionary radiation.
As mentioned many times in several contexts within this chapter, Alan
Cheetham's studies of the bryozoan Metrarabdotos (1986, 1987), now
supplemented with the work of Jackson and Cheetham (1994, and Cheetham and
Jackson, 1995) on Stylopoma, have set a standard of excellence and confidence for
empirical studies of relative frequency. All major desiderata for such research have
been realized in these genera—a group with a well-resolved phylogeny, in a clade
restricted to a geographic region, and exhaustively sampled in strata of unusually
complete resolution over a long period. Moreover, Cheetham's multivariate
morphometrics permit us to assess stasis and punctuation as a morphological
totality, not only as a potentially biased impression based on a few preselected
characters. Finally, studies with Jackson on the ecology and genetics of extant
species demonstrate (see page 786) that the morphology of paleospecies almost
surely provides a good surrogate and identifier for true biospecies in this clade. (As
a personal note, I am also gratified that Cheetham began these studies with the
intention of proving his suspicions for gradualism in the context of the developing
debate about punctuated equilibrium—and ended up with the finest data-driven
evidence